Rotative control shafts in use in television receivers and the like typically embody an extension of generally D-shaped cross-section adapted for engagement with an operating element comprising a knob having a recess of like cross-section. For securing the operating element to the control shaft, the art has looked to various retainers in the form of spring clips typically resiliently engaging the shaft extension periphery and the operating element interior. Such retainers are customarily inserted into and are entirely contained by the operating element and include a projection or like means preventing rotation and/or axial displacement thereof relative to the operating element.
Known operating element-contained retainers comprised of elongate slit sleeves of elastic sheet material formed in such D-shaped cross-section, such as are shown in U.S. Pat. No. 2,968,504, No. 2,745,689 and No. 2,080,361, incorporate the above-mentioned projection and embody spring fingers or the like extending interiorly from an arcuate part of the retainer periphery to resiliently engage the control shaft for maintaining the same in secured relation to the operating element. Such arcuate periphery part itself resiliently engages the operating element interior wall.
Apart from the foregoing common structural features, the referenced operating element-contained retainers are characterized by differences in symmetry of their construction. For instance, the latter two patents disclose retainers configured such that an exclusive one of the opposed ends thereof is first insertable in the operating element and the spring fingers thereof are longitudinally unsymmetrical, whereas either end of the retainer of the first-mentioned patent is first insertable in the operating element and the spring fingers thereof are longitudinally symmetrical.
The art has also looked to some extent to retainers engageable with the peripheries of both the shaft extension and the operating element. Thus, U.S. Pat. No. 2,733,083 discloses a retainer having a periphery comprised of an arcuate part for encircling a hollow D-shaped extension of the operating element and a straight part, operative together with the hollow D-shaped extension to envelop a shaft extension of like D-shape, and embodying a spring finger engaging the control shaft and urging the same into engagement with the operating element interior.
In performance, known retainers are considered to achieve less than desired performance levels. In respect of the first-mentioned operating element-contained retainers, this consideration is based on the apparent dependence thereof on quite close tolerance required of the diameter of the control shaft extension, particularly insofar as the constancy of such diameter defines the shaft extension periphery engaged by the retainer spring fingers. In respect of the retainer disclosed in the last-mentioned patent, this consideration is based on the apparent dependence thereof on quite close tolerances required of both the control shaft extension diameter and the diameter of the operating element extension. This latter arrangement is further considered to be less than desired as requiring tooth and groove engagement as between the operating element exterior periphery and the retainer and in appearance since the retainer element is not contained by the operating element but is exposed thereon.
Other considerations arise in respect of forces involved in inserting the control shaft extension into the assembled operating element and retainer and in extracting the shaft from the assembled operating element and retainer. In some instances, the maximum allowable insertion force is determined largely by the nature of the mechanism controlled by the control shaft. Where such mechanism is relatively delicate, the maximum allowable insertion force may be limited to as low as 15 pounds. To maximize extraction force given such 15 pound upper limit, the retainer desirably should exhibit a high ratio of extraction force to insertion force. Known retainers are considered not to exhibit this ratio in as high a magnitude as is desired, thus providing lesser than desired extraction force.
A further observation concerning known retainers relates to the matter of the stress induced in their spring fingers as the control shaft applies bending moments thereto. In this respect, known retainers do not define spring finger structure facilitating equal distribution of bending moments and equalization of stress as between the spring fingers.